The present invention relates to a brake device of a transmission to be mounted in a vehicle, and also relates to a technical field of transmissions for vehicles.
Automatic transmissions mounted in vehicles generally switch a drive force transmission path formed by planetary gear sets, etc., by selectively engaging a plurality of hydraulic friction engaging elements, and automatically set a gear position. While the gear position is basically formed by engaging two of the friction engaging elements, a first gear position in a drive range is conventionally formed by a single friction engaging element and an OWC (one-way clutch) to smoothen the gear shifting operation.
However, the price of the OWC is high. Moreover, the OWC operates as a rolling resistance in gear positions other than the first gear position in the drive range, which prevents an improvement in the fuel consumption performance of the engine and a reduction the power consumption of an electric motor in a hybrid vehicle and the like. Therefore, recently, the elimination of the OWC has been discussed.
In this case, for example, the first gear position is formed by engaging a low clutch and a low reverse brake. The low clutch is engaged at predetermined low gear positions including the first gear position. The low reverse brake is engaged at the first gear position and the reverse gear position. Shifting to the first gear position is performed by engaging the low reverse brake while the low clutch is engaged. Therefore, to smoothly shift to the first gear position from a different gear position, it is necessary to precisely control a timing of engaging the low reverse brake and an engaging force, with high responsiveness.
As a solution to the above disadvantage, JP2005-265063A discloses a brake device for a low reverse brake using a tandem hydraulic actuator having two pistons.
As illustrated in FIGS. 13A and 13B, a brake device 500 includes a plurality of fixed friction plates 531 spline-engaged to a transmission case 510 side, a plurality of rotatable friction plates 532 spline-engaged to a rotational member 520 side, an engaging piston 550 which engages a friction plate set 530 of the friction plates 531 and 532 against a biasing force of a return spring 540, and a clearance adjusting piston 560 disposed on a back side of the engaging piston 550. The friction plates 531 and 532 are alternately disposed between the transmission case 510 and the rotational member 520 accommodated inside the case 510.
In the brake device 500, as illustrated in FIG. 13A, the pistons 550 and 560 are kept at their retreat positions by the biasing force of the return spring 540 in an unengaged state, and a comparatively large clutch clearance (a total dimension of a gap formed between an engaging piston and a member which receives a pushing force of the piston, in a release state of the brake device) is formed. In this state, when a hydraulic pressure is supplied into a clearance adjusting hydraulic chamber 570 formed on a back side of the clearance adjusting piston 560, as illustrated in FIG. 13B, the pistons 550 and 560 move forward against the biasing force of the return spring 540 until the clearance adjusting piston 560 reaches an end of the stroke, and the clutch clearance is accordingly reduced. Thus, a so-called small clearance state can be obtained.
Therefore, by supplying the hydraulic pressure into the engaging hydraulic chamber 580 on the back side of the engaging piston 550 in this state, the brake device 500 is engaged with high responsiveness, and the timing of the engaging operation and the engaging force can be precisely controlled.
Moreover, in a state where the brake device 500 is released, since the comparatively large clutch clearance is formed, a resistance that acts on the rotational member 520 due to a viscosity resistance of a lubricant between the fixed friction plate 531 and the rotatable friction plate 532 becomes small, and degradation of the fuel consumption performance of the engine and the like can be reduced.
Meanwhile, in the brake device 500 of JP2005-265063A, as illustrated in FIGS. 13A and 13B, the engaging hydraulic chamber 580 is formed by fitting the engaging piston 550 into a first concave portion 512 formed in a vertical wall 511 of the transmission case 510. The clearance adjusting hydraulic chamber 570 is formed by fitting the clearance adjusting piston 560 into a second concave portion 513 formed on the back side of the first concave portion 512 of the vertical wall 511.
Further, when the hydraulic pressure is supplied into the clearance adjusting hydraulic chamber 570, the clearance adjusting piston 560 inside the second concave portion 513 of the transmission case 510 moves rearward (leftward in FIGS. 13A and 13B), and an end of the clearance adjusting piston 560 projects into the first concave portion 512 and contacts with the engaging piston 550 to move the engaging piston 550 to the friction plate set 530 side. Thus, the clutch clearance is reduced. However, due to the engaging piston 550 inside the first concave portion 512 moving rearward, a volume of the engaging hydraulic chamber 580 on the back side thereof becomes large.
When the volume of the engaging hydraulic chamber 580 is increased as above in adjusting the clutch clearance, the responsiveness to engage the brake device 500 when the hydraulic pressure is supplied into the engaging hydraulic chamber 580 degrades, and the improvement effect of the responsiveness by reducing the clutch clearance may be lost.
This disadvantage is not limited to arising in an automatic transmission, but it also arises similarly in other kinds of transmissions, such as continuously variable transmissions including a brake device having a similar configuration.